Taxonomy & naming
The dwarf cory was described in 1888 by Carl and Rosa Eigenmann as Corydoras hastatus, from material taken at Villa Bella — now Parintins, on the Amazon between Santarém and Manaus, in Amazonas state, Brazil, which remains the type locality. A Paraguayan population was later named Corydoras australe by Eigenmann and Ward in 1907, but the two could find little to separate them beyond a sharper black lateral stripe in Amazonian fish, and australe is today treated as a junior synonym of hastatus. The species sits in family Callichthyidae, the South American armoured or mailed catfishes.
For more than a century it carried the name Corydoras hastatus, and that is still how it is traded and discussed. The genus Corydoras, however, had swollen past 170 species and was shown by molecular work to be unnatural. In 2024 Dias, Tencatt and colleagues published a large phylogeny and revised classification of the Corydoradinae (DOI 10.1093/zoolinnean/zlae053) that split the old genus among several names — Brochis, Hoplisoma, Osteogaster, Gastrodermus and others. Under that scheme the true dwarf corys, hastatus among them, fall in lineage 5 and take the resurrected genus Gastrodermus, so the current Catalog of Fishes combination is Gastrodermus hastatus (Eigenmann & Eigenmann, 1888) — and FishBase and Eschmeyer's Catalog of Fishes both now head the species that way. The change is recent and far from universal in the hobby, which still says 'Corydoras hastatus,' so we keep that as the heading while recording Gastrodermus as the accepted genus.
Morphology
This is a true miniature: maximum standard length is only about 1–1.5 in, making it one of the three smallest corys alongside C. pygmaeus and C. habrosus. Like all members of the family it is armoured rather than scaled, with two rows of overlapping bony plates (scutes) down each flank — about 22 in the upper row and 20 in the lower — meeting along the back, and a small blunt head bearing the usual three pairs of sensory barbels around the mouth.
The body is a translucent silvery-grey, often with a faint dark mid-lateral line, and the single diagnostic mark is a black, roughly diamond- or arrow-shaped spot on the caudal peduncle, usually edged front and back with white or cream — the 'tail spot' and the source of the epithet hastatus, 'bearing a spear'. It is easily told from the otherwise similar C. pygmaeus, which lacks the tail spot but has a bold dark lateral stripe instead. The fins are short, 1 spine and 7–8 soft rays in the dorsal, 2 spines and 5–6 soft rays in the anal. What sets hastatus apart anatomically is a suite of features pointing to a life off the bottom: a relatively large eye, a more terminal mouth, a more deeply forked tail and a more symmetrical, less bottom-flattened body than its sediment-grubbing relatives.
Habitat
Corydoras hastatus has an unusually wide range for the genus, spanning two great river systems. It is recorded from the Amazon basin — the main channel near the type locality, the rio Urubu and floodplain lakes around Manaus, and downstream into Pará — and across northern and eastern Bolivia in the Madeira drainage (the ríos Iténez/Guaporé, Yacuma, Itonomas and Piraí), then far to the south throughout the Río Paraguay system in Mato Grosso do Sul, Paraguay and north-eastern Argentina, at least to the Paraguay–Paraná confluence.
It is a fish of still and sluggish water: marginal channels, backwaters, swamps, floodplain lakes, ponds and small tributaries, shallow and clear-to-turbid, over mud and clay. Some of these waters are bare; many are choked with submerged grasses or floating plants such as Eichhornia (water hyacinth) and Pistia, among which the shoals shelter and feed. Reported water conditions are warm and broad — roughly 72–82 °F, pH about 6.0–8.0 and soft to moderately hard (around 5–19 dH). Like other corys it is a facultative air-breather, gulping surface air and absorbing oxygen across a modified, vascularised hindgut, which lets it persist in the warm, oxygen-poor floodplain pools it favours.
Feeding
The dwarf cory is a foraging omnivore, but with a twist that follows from its lifestyle. Where most corys work the substrate exclusively, hastatus spends much of its time in midwater and is partly adapted to pick pelagic zooplankton from the water column as well as sifting the bottom. In captivity it takes small sinking dried foods, but it does noticeably better with a steady supply of small live and frozen fare — Daphnia, newly hatched and adult brine shrimp (Artemia), and finely chopped bloodworm — sized to its tiny mouth. Because it is so small it competes poorly for food with larger tankmates, and it should never be expected to live on the leftovers of bigger fish or to 'clean' a tank; a dedicated, frequent feeding of suitably small foods is what keeps a shoal in condition.
Mating
Corydoras are egg-layers with the celebrated 'T-position' courtship and no pair bond, and hastatus follows the pattern at miniature scale. When a female is ripe the shoal becomes active and a male presses against her head at right angles, forming the T; she appears to take his milt, which passes quickly through her gut, while she clasps just two to four eggs in a basket made by her pelvic fins. She then carries the tiny clutch to a cleaned surface — aquarium glass, a fine-leaved plant, a spawning mop — and presses the very sticky eggs into place. The sequence repeats, a few eggs at a time, until something on the order of a hundred eggs have been laid; some accounts suggest the species may sometimes deposit eggs almost singly.
Breeding
Dwarf corys can be bred along the lines of other Corydoras, though raising the minute fry is the real test. Condition a group well on live foods, skewing the sex ratio to two or more males per female, and when the females are visibly full trigger spawning with a large (50–70%) water change using cooler, soft water, with extra oxygenation and flow — repeated daily until they spawn. Eggs are scattered on the glass, fine plants or a spawning mop, the last making them easy to lift; the adults give no care and will eat eggs, so eggs or parents are separated. Most breeders move the eggs to identical, well-oxygenated water with a little methylene blue or an alder cone against fungus. Hatching takes about three to four days, and once the yolk sac is absorbed the fry take microworm and Artemia nauplii. They are delicate, demanding excellent water quality, but seem to fare better raised over a thin layer of sand than on bare glass.
In the aquarium
This is a delightful nano-aquarium fish, but its size dictates the rules. It is intensely social and should be kept in a proper shoal — at least six, and far better ten or more — when it will school and hover in the open in a way few other corys do; kept in ones or twos it pines. Despite its small body it is an active midwater swimmer and wants floor space rather than a tiny cube: a base footprint of around 60 by 12 in is a sensible minimum, and it is not really a fish for the smallest 'nano' tanks.
Provide soft sand or very smooth fine gravel kept scrupulously clean — corys feed by sifting and coarse or dirty substrate erodes their barbels — with driftwood, leaf litter and some planting, including surface cover, for security. Keep the water warm but not hot (about 72–79 °F), gentle in flow, well-filtered and well-oxygenated, across a broad pH and hardness band. Tankmate choice matters more than usual: hastatus is utterly peaceful but easily out-competed or eaten, so pair it only with small, calm fish — pygmy rasboras, small tetras, dwarf shrimp and the like — never anything large enough to bully or swallow it. As with all corys, mind the stiff pectoral spines, which can sting and tangle in a net, and transport them gently, since a stressed cory can release skin toxins that, in the small volume of a bag, can wipe out the whole group.
Conservation
Corydoras hastatus is assessed by the IUCN as Least Concern (assessment dated 2021). It is widespread across two major basins, ecologically flexible and tolerant of the still, vegetated floodplain habitats that remain common, so as a species it is under no immediate threat, and aquarium specimens are increasingly tank-bred rather than wild-caught. That does not exempt it from the broad pressures on South American freshwaters — deforestation, agriculture, damming and pollution all degrade the marginal and floodplain waters it lives in — but its wide range and adaptability leave it secure for now. As with much of the old genus, the more interesting footnote is taxonomic: the reshuffling of these dwarf species into Gastrodermus is recent, and the full extent of cryptic diversity within the broadly-defined hastatus has not been fully resolved.